Fluorescent lamps operate more efficiently at higher frequencies than at power frequencies. Operating circuits for fluorescent lamps, thus, frequently use self-oscillating circuits, having active switching elements, such as transistors, which are supplied with a control signal or with control energy to control the active switches to change state at the desired operating frequency. Circuits to supply such low pressure discharge lamps usually used a separate current transformer which was constructed either as a saturated current transformer or a transformer having an air gap. Such separate current transformers were constructed to interrupt furnishing a control signal after a predetermined voltage-time integral had elapsed. In the gap between switching pulses, sufficient time is available to clear the base or gate circuit of charge carriers, to ensure that the switching elements are reliably disconnected. Circuits of this type are described in the referenced U.S. Pat. Nos. 5,008,596, Kastl et al; 4,710,682, Zuchtriegel.
The publication by Siemens AG, "Elektronikschaltungen" ("Electronic Circuits") by Walter Hirschmann, chapter "Electronic Accessory Apparatus for New Fluorescent Lamps", pp. 147-151, is a general literature reference for the circuits described.
Circuits of these types require a current transformer for control of the active switching elements. Such a current transformer or similar element is comparatively expensive since it requires a separate winding or wound structure. Ferrite cores are used for the transformer windings. Such ferrite cores have manufacturing tolerances which undesirably and substantially affect the operating parameters when used in saturated transformers.